We have implemented Schottky barrier GaN ultraviolet detectors, both in vertical and in lateral configuration. The devices exhibit a high gain at both reverse and forward bias. The gain mechanism is attributed to trapping of minority carriers at the semiconductor-metal interface, with excellent agreement between the calculated responsivity and the experiment. However, the vertical detector zero-bias responsivity (without gain) is two orders of magnitude higher. This is attributed to improved ohmic back contacts, due to the highly doped buried layer.Schottky barrier ultraviolet (UV) detectors have been an attractive photovoltaic structure, providing high response, low noise, fast response time and easy fabrication. Ideal photovoltaic devices are not expected to exhibit gain. One of the issues yet unexplained in GaN based Schottky UV detectors, has been the presence of gain [1,2]. Presently, the III-nitride films are grown hetero-epitaxially on mismatched substrates [3]. This results in a rich microstructure dominated by dislocations that traverse vertically from the layer-substrate interface, all the way up to the film surface [4]. Typical dislocation density in GaN epitaxial layers is 10 8 -10 11 cm --2 [5]. We have recently demonstrated that the preferential orientation of the dislocations manifests itself in an intrinsic anisotropy in transport properties of the film [6]. Although lateral and vertical geometry GaN-based Schottky detectors have been already fabricated [1,2], no comparison has been made between the two detector configurations.In this work we explain the gain mechanism in GaN based Schottky detectors, and its bias dependence, for both detector structure types. We show that the vertical detector exhibits higher responsivity, lower 1/f noise, resulting in better detection properties.The layers used were grown by low-pressure metal organic chemical vapor deposition (MOCVD). For details see Ref.[7]. The vertical Schottky barrier detector (Fig. 1b) was fabricated by defining %300 mm diameter circular mesas. On the n + -GaN layer around the mesas, ohmic contacts were evaporated, using Ti : Al : Ni : Au (15, 220, 40, and phys. stat